The commercial sewing industry has found double lock stitch sewing machines attractive for work that requires continuous or long duration machine operation. For example, in the making of automobile upholstery components, a double lock stitch machine can be run continuously as successive blanks of upholstery elements are fed to the machine for creating such features as the upholstery rolls. Because double lock stitch machines use large thread spools instead of the limited capacity bobbins required by lock stitch; i e., federal specification 301 type stitch, machines, the double lock stitch machines can be operated for long periods since interruption for replacement of the thread supply is significantly reduced.
Certain problems, however, do exist with double lock stitch sewing. Due to the intertwined nature of a double lock stitch, it has a tendency to unravel at the end of a seam, or length of stitching when the thread chain is cut and the work removed from the machine. The effect is to create an unsightly and potentially defective seam or line of stitching. Further, this problem becomes more severe as the length of the double lock stitch is increased; i.e., decreased stitch density, as for example, in automobile upholstery work.
In the past a number of approaches have been proposed to reduce the severity of this problem. For example, it has been proposed to extend the seam or stitching past the end of the fabric so as to form a "tail" to thereby reduce the chance unraveling will progress to the sewn fabric. This approach, however, is wasteful of thread and tends to create unsightly finished products having treads dangling from them. Additionally, the tail is susceptible to being caught and pulled raising the possibility of opening the seam or stitching. While it has been proposed to sew the tail back into the fabric, or back stitch the seam for a prescribed length, these approaches are likewise undesirable because they complicate production with extra steps and can require use of specialized machines or at least specialized machine attachments.
It has been suggested that a more desirable solution to the problem of unraveling could be provided if stitch length were changed during operation. Specifically, it has been proposed to make the double lock stitch more dense; i.e., decrease the stitch length, at the end of a seam or line of stitching. It has been found that due to the nature of the double lock stitch, the tendency for unraveling is substantially reduced if length is reduced; i.e., made more dense. It is believed that the decreased spacing between the locking portion of the stitch in a denser stitch reduces the ability of the thread to undo itself and release the stitch locks.
But, the ability to change stitch length during operation requires specialized double lock stitch sewing machines. As described in U.S. Pat. No. 4,643,113, issued Feb. 17, 1987 to Schrudde et al. and U.S. Pat. No. 3,742,880 issued July 3, 1973 to Franz, double lock stitch machines can be designed having mechanisms that permit stitch length to be manually adjusted without need for interrupting machine operation.
However, in conventional double lock stitch machines such as the Singer.RTM. 300W type machines widely used in industrial sewing today, stitch length can not be changed during machine operation. Rather, in such machines, adjustable eccentrics located within the machine at the arm and main drive shafts are required to be manipulated to change stitch length. As a result, it is not possible to change stitch length in such machines without stopping the machine and, thus interrupting production, a commercially undesirable procedure.
Accordingly, it is an objective of this invention to provide apparatus that may be used in double lock stitch sewing machines lacking dynamic stitch length adjustability for enabling the length of the machine stitch to be changed during operation.
It is a further object of this invention to provide apparatus that may be used in double lock stitch sewing machines lacking dynamic stitch length adjustability for enabling the length of the machine stitch to be reduced during operation at least at the end of a seam or line of stitching.
It is a still further object of this invention to provide a double lock stitch sewing machine having a stitch length that may be changed during operation.
It is a yet a further object of this invention to provide a double lock stitch sewing machine having a stitch length that may be reduced during operation at least at end of a seam or length of stitching.
It is still another object of this invention to provide apparatus that may be retrofitted to double lock stitch machines lacking dynamic stitch length adjustability for enabling machine stitch length to be varied at will during operation.